Wireless communication systems are widely deployed to provide various types of communications such as voice, data, video, etc. These systems may be multiple-access systems capable of supporting communication with multiple access terminals by sharing available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, orthogonal frequency division multiple access (OFDMA) systems or SC-FDM. Typically, a wireless communication system comprises several base stations, wherein each base station communicates with a mobile station using a forward link and each mobile station (or access terminal) communicates with base station(s) using a reverse link.
Systems based on CDMA are generally more robust in comparison to FDMA systems as they can flexibly increase spreading codes on channels in accordance with bandwidth requirements. Therefore, unlike FDMA systems, they allow channels to be reused among adjacent cells/sectors. However, such channel reuse can decrease capacity of systems besides causing interference at cell/sector boundaries that share the channels. Hence, while CDMA may effectively deliver many low data rate signals like mobile voice, this technology may not be well suited for simultaneous delivery of high speed signals such as broadband data.
OFDM based systems are more effective in dealing with multipath and frequency selective fading in a broadband channel. A frequency selective channel occurs when a transmitted signal experiences a multipath environment where a given received symbol can be potentially corrupted by a number of previous symbols. This phenomenon is generally known as inter symbol interference (ISI). OFDM is based on the idea of frequency-division multiplexing (FDM), which involves sending multiple signals at different frequencies. An OFDM baseband signal is a sum of a number of closely-spaced orthogonal sub-carriers. By utilizing orthogonal frequencies, the sub-carriers within an OFDM system may actually overlap without interfering with each other thereby achieving greater spectral efficiency as compared to FDM. While OFDM systems facilitate servicing several users simultaneously by assigning different sets of orthogonal sub carriers to different users, they suffer from high PAPR (Peak to Average Power Ratio) leading to lower power efficiency. This disadvantage can be overcome by a modified version of OFDM for uplink transmissions in the “long-term evolution (LTE)” of cellular systems called single-carrier FDM (SC-FDM).
SC-FDM systems are similar to OFDM systems as they use different orthogonal frequencies (sub-carriers) to transmit information symbols. However, in contrast to OFDM systems, the information symbols first go through a DFT transformation/spreading before going through the tone mapping and IFFT. This operation reduces the fluctuations in time domain and leads to lower PAPR. Within SC-FDM systems, sub-carriers can be distributed among terminals in accordance with different methods. One method known as localized SC-FDM (LFDM) involves assigning a contiguous set of sub-carriers to a user equipment (UE) to transmit its symbols. Another method is known as interleaved FDM (IFDM) where occupied sub-carriers are equidistant from each other. However, due to various factors SC-FDM can restrict operations which necessitates communication systems/methods that can provide flexibility while optimizing power usage.